1. Field of the Invention
The present invention relates to a sheet processing apparatus, which performs post-processing to a sheet bundle formed of a plurality of sheets having an image formed thereon in an image forming system such as a copying machine, a multifunction peripheral and the like.
2. Description of the Related Art
Conventionally, a sheet processing apparatus (post-processing apparatus: finisher) which performs an aligning process, a binding process and a punching process are known. In the aligning process, a plural number of sheets discharged from the image forming apparatus are received onto a processing tray. The received sheets are bound into a sheet bundle. By the binding process and the punching process, a predetermined portion of the sheet bundle is bound. The post-processing function of the sheet processing apparatus limits the number of sheets of the sheet bundle which can be post-processed. One of the reasons for this is because there is a limit of an opening height of a binding section of a stapler.
On the other hand, before a document reading apparatus finishes reading of the all pages, the image forming apparatus sequentially starts to form image of a document having finished reading. Then, the image forming apparatus promptly outputs the sheet on which an image has been formed. It means that, the sheet processing apparatus receives sheets before a page configuration of the sheet bundle (for example, the number of sheets) subject for the post-processing is determined.
For example, description is made in a case where it is determined in the middle of forming the sheet bundle that the number of sheets exceeds the upper limit number of sheets which can be post-processed. In this case, the sheet processing apparatus does not perform the post-processing to the sheet bundle so as not to interrupt the operation of the apparatus. Instead, the sheet processing apparatus discharges the sheet bundle to a stack tray even if not all sheets of the sheet bundle have been received.
In this case, however, the sheet bundle which was post-processed and completed normally (normal bundle) and the sheet bundle which was not post-processed due to the excess of the upper limit number of sheets (exceptional bundle) are discharged at the same position. Therefore, there is a problem that the discharged sheet bundles cannot be distinguished respectively.
Japanese Patent Laid-Open No. 2000-016683 describes an image forming system to this problem. In the image forming system disclosed in Japanese Patent Laid-Open No. 2000-016683, a stack tray (shift tray) adapted for shift operation is employed. The stack tray enables to shift a stack position when the sheet bundle having failed to be post-processed is discharged as an unprocessed sheet bundle to sort the respective discharged bundles.
On the other hand, in a sheet processing apparatus such as an inner finisher which is placed in a space between a printing part and reader part of the image forming apparatus, it is not easy to provide a shift tray whose configuration easily interferes with peripheral devices.
In addition, in order not to deteriorate productivity in image formation, the post-processed sheet bundles are discharged and stacked to the position at which the sheet bundles are post-processed or close thereto. This helps to reduce time required to change positions such as discharge position. This is also applied in case of the exceptional bundle. For example, in a case where the exceptional bundle is occurred while running a designated job of “depth binding process”, which is a post-processing performed at a depth position when viewed from the front of the image forming apparatus, the exceptional bundle is placed at a depth position on a discharge tray. Therefore, this leaves a problem that, in an inner finisher having a small open space, when a user checks the sheet bundle discharged onto the stack tray, the visibility of the sheet bundle is decreased